A colorlithographic printing process involves the transfer onto an image receptor sheet of a plurality of different color components of the required image. The ink for each successive image component is transferred by an offset process, the initial ink pattern being formed on the surface of a specially prepared lithographic plate in the form of a thin metallic sheet secured onto the surface of a carrier roller. To ensure that the successive image components are transferred with exact positional accuracy to provide a sharp composite image, it is known to provide the printing machine with means to locate the printing plates correctly on their respective support rollers. Such locating means normally comprises projections which engage in preformed registration apertures in the printing plates. The equipment used by the printing plate maker is suitably adapted to punch these registration holes in an accepted standard format according to the type of printing machine for which the plates are intended, and also to form the respective color component print patterns in correct positional relationship with these punched registration holes.
For transport from the plate maker to the printer, each set of plates is assembled as a stack and secured together. This is normally done using pieces of adhesive tape secured at the corners of the stack, but this somewhat crude technique has certain drawbacks. Firstly, the tape must be very carefully applied otherwise the plate may not be sufficiently firmly secured together to prevent face-to-face sliding of the plates which causes damage to the image patterns carried on the faces of the plates. Accordingly, this adhesive tape method is time-consuming and awkward to perform. Secondly, the tape often splits during transport, allowing such sliding to occur, with the above-noted undesirable results. Thirdly, the tape must be completely removed by the printer otherwise printing errors may occur; this tape removal is again a time-consuming and awkward job to perform.
After printing, the printer will normally store the printing plates, and once again each set of plates may for convenience of storage be secured together, for which purpose the above-mentioned troublesome adhesive tape method is normally employed.
The process used by the plate maker for making the printing plates is a photographic process involving the use of sheets of photographic film. These sheets, or transparent carrier sheets on which they are mounted, are also punched with registration holes, to ensure correct relative positioning of the plates and films during the process, and the above-noted problems of securing the sets of films together for transport and storage are commonly encountered.